The present disclosure discloses a device that efficiently mixes two flowing combustion gas streams and reduces gas backpressure under varying furnace loads.
It is common to mix gases of different temperatures in many applications, such as in boilers or steam generators. For example, all hot flue gases are passed through an economizer when the furnace of a boiler is operating at full load. The economizer recovers heat from the flue gases to preheat feed water that is circulated back into the boiler. The resulting flue gases exiting are cooler, due to the heat transfer. Therefore, the heat is recycled into the boiler, increasing boiler efficiency.
However, when the furnace/boiler is operating at a low load, if all of the flue gases are passed through the economizer, the temperature of the flue gases may drop below a critical temperature required for certain chemical processes, such as the catalytic removal of NO, NO2 (collectively referred to as NOx) from the flue gases in a selective catalytic reduction (“SCR”) system. Since the catalytic reactions are temperature dependent, the SCR must function within a specified temperature range in order to satisfactorily perform its required function.
Therefore, in low boiler load conditions, only a portion of the flue gases should pass through the economizer, and the remainder should bypass the economizer to maintain a higher temperature. These two gas streams are then mixed to result in flue gases within a required temperature range.
One way to regulate flue gas temperature is through the use of dampers and bypasses. A flue gas bypass allows a portion of the flue gas stream to bypass the economizer, with the remaining portion of flue gas stream being routed through the economizer. The streams are then mixed to result in a mixed stream that has a higher temperature than if all of the flue gases passed through the economizer.
Conventional boilers employ mixers with angled, fixed vanes. These mix the gas, but produce a pressure drop at all boiler loads. This pressure drop requires larger, more expensive fans and increased auxiliary power consumption.
Conventional mixers require a certain time period to mix the gases, under a given boiler load. This equates to a certain duct length (transition section) to sufficiently mix the flowing gas streams. There may be an excess of high temperature gases in contact with the surface of the flue duct causing ‘hot spots’. The ‘hot spots require high temperature metals, which are typically more expensive than standard metal. Longer transition sections add to the costs of the system. It would be beneficial to employ a device that would mix two flue gas streams more quickly, and after a shorter length down the flue gas duct. This would then shorten the transition section, thereby requiring less high temperature metal to construct the device.
Currently, there is a need for a simple and inexpensive device for mixing gases more efficiently that reduces the backpressure under various boiler loads.
The disclosure may be understood more readily by reference to the following detailed description of the various features of the disclosure and the examples included therein.